Why We Turn Up Our Noses

The odor of rotting vegetables disgusts most of us, and for good reason: Eating bad food can make us sick. Now scientists have tracked this inborn disgust back to its roots--the amygdala, a part of the brain that processes emotions. Experts say the findings, reported in the current issue of the Proceedings of the National Academy of Sciences, are the first evidence linking a brain region with an odor-triggered emotional response.

David Zeld and Jose Pardo of the Veterans Affairs Medical Center in Minneapolis exposed a dozen women to sulfurous compounds that various subjects described as smelling like "rotting vegetables," "a sewer," and "garlic breath." As the women took deep whiffs of the foul vapors, the researchers measured blood flow to the brain with a technique called positron emission tomography. The blood flow increased dramatically to both the left and right lobes of the almond-sized amygdala, just as the women reported feeling fearful, tense, and repulsed. When the women were presented with an odorless sample, blood flow levels remained steady and they reported feeling calm.

Zeld speculates that the amygdala may be coding the sulfurous odors as "bad" and translating the stimuli into feelings of fear or disgust. Other researchers have been unable to trigger such a robust neurological response in subjects looking at gory photographs or listening to screeching sounds. Says Pamela Dalton, a researcher at the Monell Chemical Senses Center in Philadelphia, "Finding that there is a strong correlation between a subjective response and brain activity is very, very exciting."